Quenched and tempered martensitic steels are currently broadly used to produce high strength seamless pipes for OCTG applications.
One interesting alternative to get improved mechanical properties is the use of carbide-free bainitic steels in the as rolled or as rolled and tempering conditions. The chemical composition of these steels must be carefully designed to suppress the ferrite and pearlite reactions during the slow air cooling from the austenitic range after hot rolling.
The loss of toughness and ductility commonly observed in bainitic steels is usually related to the presence of coarse cementite particles between the bainitic ferrite sheaves. In order to avoid this problem, it was proposed to inhibit the cementite formation by the addition of more than 1 wt % of Silicon or Aluminum. These elements can not be dissolved in cementite, and hence suppress its precipitation.
From the document WO96/22396 there is known a carbide-free high Si/AI bainitic steel, but it is used for different applications than for OCTG applications. In particular WO 96/22396 discloses a method of producing a bainitic steel product, whose microstructure is essentially carbide-free, comprising the steps of: hot rolling the steel product and either cooling the steel from its rolling temperature to ambient temperature continuously and naturally in air or by continuously accelerated cooling. The cooling rates used are between 225 and 2° C./s, therefore comprising very high cooling rates.
The material is produced as rolled or after accelerated cooling, and the product is always intended for different applications than for OCTG applications.
It is a fact that bainitic steels in the as rolled condition or after accelerated cooling can not be directly used for high strength OCTG applications. Due to the low yield to tensile strength ratio, the required tensile and impact properties cannot be achieved, in particular for some OCTG applications.
The need is therefore felt to provide a steel composition and a process for producing seamless steel tubes having high strength and toughness, suitable for OCTG applications.